Algae's solar electrons hijacked to steal power

An international gang of biologists has carried out an audacious heist, stealing valuable electrons from photosynthesising algae.

The power grab could open a route to more efficient exploitation of photosynthesis to power machines: with biofuels we are already converting solar power into a form that engines can use, but almost three-quarters of the sunlight energy absorbed by the organisms is lost before it can be turned into the sugars or starches used to make biofuels.

Grabbing photosynthetic energy earlier in the process should allow much more to be extracted, says WonHyoung Ryu at Yonsei University in Seoul, South Korea. "Theoretically we should be able to collect all photosynthetic electrons."

Trapped cells

Ryu worked with colleagues at Stanford University, California, to plug gold electrodes directly into algal cells and draw off electrons carrying energy absorbed from light.

The team captured the unicellular algae Chlamydomonas in tiny traps. They attached an ultra-sharp gold electrode to an atomic force microscope and inserted its 30-nanometre-wide tip into the photosynthesising organs – chloroplasts – of an algal cell. That electrode was connected to an electric current meter, and a second gold electrode was placed in the cell's growth medium to complete the circuit.

The light reaching an alga's chloroplasts is used by proteins inside to split oxygen from water, releasing electrons that are passed between other molecules to provide energy to drive chemical reactions.

When Ryu and colleagues shone a halogen lamp on their alga, those electrons were siphoned off by the electrode instead. Their circuit registered a current of 1.2 picoamps – which is equivalent to a yield of 0.6 milliamps per square centimetre. By increasing the light intensity that value rises to a maximum of 6 milliamps per square centimetre, Ryu says.

By contrast, some silicon solar cells have a current density of 35 milliamps per square centimetre. Despite that, Ryu thinks his algae could still find a job in power generation. "The solar cell efficiency is also related to the wavelength of light," he says. "We believe our bio-solar system may provide higher efficiency than the silicon-based solar cells at particular wavelengths." Chlorophyll, for instance, has evolved to absorb blue and red light well, but doesn't absorb much green light, hence its colour.

Where next?

The team thinks the gold electrode managed to snaffle around 20 per cent of the total number of photosynthetic electrons from the alga. They calculated this by comparing the number of electrons flowing through the circuit to the theoretical number that an untapped cell would use to generate oxygen under the same conditions. Improvements to electrode design should boost that figure, says Ryu.

Wim Vredenberg at the Wageningen University and Research Centre in the Netherlands has used similar techniques to study photosynthesis in algae. "[But] I never have thought of exploring electro-physiological technologies for harvesting the photoelectrons generated in chloroplasts," he says.

Biochemist James Barber at Imperial College London says the work is good, but that harvesting electrons in this way is impractical on a large scale. Using high-energy electrons that "leak" from some micro-organisms – a phenomenon exploited in microbial fuel cells – is more practical, he says.

"There is still a lot to do to make a practical system," concedes Ryu. "We thought of having an array of cantilevers that have multiple electrodes for a large-scale system," he says. Each electrode would pierce the chloroplast of a separate algal cell, held in an array of traps.

But one question remains unanswered. "We do not know for sure what effect this electron stealing will have on the life of the cells," says Ryu. "We want to keep them alive as long as possible."

If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.